oslo.messaging deprecated the rpc_backend option long ago, but it was
still being used in cinder unit test configuration fixtures. oslo.messaging
9.0.0 finally removed that option, so now the tests are failing.
This change replaces use of rpc_backend with the corresponding
transport_url option that as recommended at:
https://docs.openstack.org/oslo.messaging/latest/configuration/conffixture.html
It also removes a redundant setUp() override.
Change-Id: I7cc823e96da1fd38ba28650b72a358e792d7c892
Closes-Bug: #1796759